1. Field of the Invention
The present invention relates to a multilayer glass ceramic substrate used for high density implementation of LSI elements and more specifically to a multilayer glass ceramic substrate which can be sintered at low temperatures and to a process for producing the same.
2. Description of the Related Arts
Development of semiconductor technology require down-sizing and high-speed of electronics devices and systems. In fact, as the semiconductor elements are integrated into VLSI and ULSI of high density, very high density and fine-processing are required in the implementation technique for assembling the elements. In particular, it is requested to increase the wiring density of the substrate on which semiconductor elements are implemented so as to comply with finer pattering and higher operation speed and to lower the dielectric constant of substrate materials.
Alumina multilayer substrate has been used widely. This substrate is produced by a thick film printed multilayer technique or a green sheet lamination technique. The latter technique is more advantageous so as to satisfy requirement of high-density integration. In the green sheet lamination technique, a plurality of thin ceramic green sheets each on which wiring lines are printed are laminated before integrated, so that it is easy to increase the wiring layers to a desired number and hence the wiring density can be increased comparing to the thick film printed multilayer technique.
Alumina ceramic, however, has such a demerit that sintering must be carried out above 1,500.degree. C. which require to use, as wiring conductor, Mo, W metal having a relatively high electric resistance and hence it is difficult to realize fine wiring. Still more, the dielectric constant of alumina is about 10 which is too high for high-speed operation of signals.
Recently, ceramic materials which can be sintered at relatively lower temperatures have been developed so that low resistance conductors such as Au, Ag--Pd, Ag or Cu can be used. For example, a composite material consisting of alumina and borosilicate lead glass can be sintered at a low temperature below 1,000.degree. C. to produce a multilayer substrate in which Au, Ag--Pd or Ag can be used as a wiring conductor. However, in this composite material, it is difficult to use wiring of Cu which is a base metal because the composite material contains lead and hence sintering can not be carried out in a reduction atmosphere. Still more, the electric constant of this composite material can not be lowered below 7.5.
Glass ceramic material using borosilicate glass is also know. This glass ceramic material can be sintered at a temperature lower than 1,000.degree. C. in a reduction atmosphere and has lower dielectric constant of about 5.5, so that a multilayered structure having wiring lines of Cu can be realized simultaneously at the sintering of glass ceramic. Known glass ceramic material, however, possess very poor mechanical strength because no crystallization occur by the sintering.
The mechanical strength is a very important factor of the substrate. In fact, in the case of a multi-chip implementation substrate on which a large number of semiconductor elements are implemented, the substrate size increases and input/output terminals or pins are connected at different levels, if the he mechanical strength of the substrate is poor, problems of breakage of the substrate, junction failure or the like occur in assembly stage and on a product.
An object of the present invention is to resolve the problems of conventional implementation substrate and to provide a multilayer glass ceramic substrate which can be sintered at a low temperature below 1,000.degree. C. in neutral and reduction atmosphere in addition to oxidation atmosphere and which has a low dielectric constant and improved mechanical strength.
The multilayer glass ceramic substrate according to the present invention can be used as an implementation substrate for high-density fine-wiring and high speed, since low resistance metals such as Au, Ag, Cu, Ag--Pd or the like can be used as wiring conductor.